The present invention concerns an optimization of the manufacture of phthalic acid anhydride (PA) to increase the yield of phthalic acid anhydride and to extend the life of the catalyst by combining a process step of the partial oxidation of o-xylene in the gas phase over solid bed catalysts that contain vanadium oxide and titanium oxide at temperatures below the optimum salt bath temperature, with a purification process step of countercurrent crystallization.
Processes for the manufacture of PA are known to the man skilled in the art and substantially consist in the gaseous oxidation of o-xylene where the o-xylene/air mixture is passed through tube assembly reactors filled with catalysts that contain vanadium oxide and titanium oxide. In addition to the exothermic oxidation to form PA, undesired by-products are produced, such as carbon monoxide and carbon dioxide, as a result of excess oxidation and also, for example, tolulyl aldehyde and phthalide as a result of insufficient oxidation. The underlying problem of the industrial manufacture of PA was, therefore, so to select the reaction parameters that excess or insufficient oxidation are avoided as far as possible. Particular attention was paid to keeping the phthalide content of the PA product as low as possible as this impurity presents special difficulties in the purification of PA. The reaction conditions over the catalyst are so selected that the lowest possible phthalide contents are obtained in the reaction product. The purity of the product is correlated with an increase in the temperature of the salt bath in the reactor. The optimum salt bath temperature is usually that temperature at which PA with phthalide contents of from 0.01% to 0.1% by weight are obtained in the crude product. This means, however, that the temperature stress reduces the life of the catalysts in the long term and the presence of such small amounts of the insufficiently oxidized product, phthalide, results in excess oxidation of the o-xylene used with an overall loss in yield.
The purification of the crude PA is usually effected by refluxing with the addition of various neutralizing or oxidizing substances or by distillation. In practice, from the crude anhydride, approximately 2 parts or more of PA are lost per part of phthalide to be removed. Furthermore, purification by distillation entails considerable energy costs.